This invention relates to a communication circuit fault detector for testing a communication circuit.
The term communication circuit refers generally to a circuit that receives an input data signal at one location and reproduces a substantially identical output data signal at another location. Communication circuits are ubiquitous in data processing systems, being used for communication between separate data processing equipment, between separate printed circuit boards in the same data processing equipment, and between separate components on the same printed circuit board.
The usual method of testing a communication circuit is to perform a loop test by sending known test signals through the communication circuit, looping the test signals back to their point of origin, and comparing the returned test signals with the original test signals. Prior-art apparatus for this purpose comprises devices for generating and comparing the test signals, and a switch that connects the output side of the communication circuit to either its normal output line or a loop-back line. During data communication, the switch is set to the normal output line position. To test the communication circuit, normal data communication is halted and the switch is moved to the loop-back position; then a test signal sequence is sent through the communication circuit, returned via the loop-back line, and compared.
A problem with this system is that it is not possible to test the communication circuit during normal operation; hence tests are inconvenient to perform and faults cannot be detected promptly.